jQuery UI buttonset disabled constructor option not working

Question

I have some buttonsets that contain radio inputs and want them to be disabled initially.

Passing in the disabled option when constructing the buttonset doesn't work.

$("#a").buttonset({disabled: true});
$("#b").buttonset({disabled: true});
$("#c").buttonset({disabled: true});
$("#d").buttonset({disabled: true});

However, calling the button disable method does.

$("#a").buttonset("disable");

Does anyone know why the first piece of code doesn't work?